Cell movement is an important part of normal developmental and physiological processes (e.g. epiboly, gastrulation and wound healing), and is also important in pathologies such as tumor progression and metastasis, angiogenesis, inflammation and atherosclerosis. The process of cell movement involves alterations of cell-cell and cell-matrix interactions in response to signals, as well as rearrangement of the actin and microtubule cytoskeletons. The small GTPases of the Rho/Rac family interact with a variety of molecules to regulate the processes of cell motility, cell-cell adhesion and cell-matrix adhesion. Cdc42 and Rac are implicated in the formation of filopodia and lamellipodia required for initiating cell movement, and Rho regulates stress fiber and focal adhesion formation. Rho/Rac proteins are effectors of cadherin/catenin-mediated cell-cell adhesion, and function downstream of integrins and growth factor receptors to regulate cytoskeletal changes important for cell adhesion and motility.
There are five members of the Rho/Rac family in the C. elegans genome. rho-1 encodes a protein most similar to human RhoA and RhoC, cdc-42 encodes an ortholog of human Cdc42, and ced-10, mig-2 and rac-2 encode Rac-related proteins. ced-10, mig-2 and rac-2 have partially redundant functions in the control of a number of cell and axonal migrations in the worm, as inactivation of two or all three of these genes causes enhanced migration defects when compared to the single mutants. Furthermore, ced-10; mig-2 double mutants have gross morphological and movement defects not seen in either single mutant, possibly as a secondary effect of defects in cell migration or movements during morphogenesis. These defects include a completely penetrant uncoordinated phenotype, as well as variably penetrant slow-growth, vulval, withered tail, and sterility defects, none of which are seen in either single mutant.
Beta-catenin is an adherens junction protein. Adherens junctions (AJs; also called the zonula adherens) are critical for the establishment and maintenance of epithelial layers, such as those lining organ surfaces. AJs mediate adhesion between cells, communicate a signal that neighboring cells are present, and anchor the actin cytoskeleton. In serving these roles, AJs regulate normal cell growth and behavior. At several stages of embryogenesis, wound healing, and tumor cell metastasis, cells form and leave epithelia. This process, which involves the disruption and reestablishment of epithelial cell-cell contacts, may be regulated by the disassembly and assembly of AJs. AJs may also function in the transmission of the ‘contact inhibition’ signal, which instructs cells to stop dividing once an epithelial sheet is complete.
The AJ is a multiprotein complex assembled around calcium-regulated cell adhesion molecules called cadherins (Peifer, M. (1993) Science 262: 1667-1668). Cadherins are transmembrane proteins: the extracellular domain mediates homotypic adhesion with cadherins on neighboring cells, and the intracellular domain interacts with cytoplasmic proteins that transmit the adhesion signal and anchor the AJ to the actin cytoskeleton. These cytoplasmic proteins include the alpha-, beta-, and gamma-catenins. The beta-catenin protein shares 70% amino acid identity with both plakoglobin, which is found in desmosomes (another type of intracellular junction), and the product of the Drosophila segment polarity gene ‘armadillo’. Armadillo is part of a multiprotein AJ complex in Drosophila that also includes some homologs of alpha-catenin and cadherin, and genetic studies indicate that it is required for cell adhesion and cytoskeletal integrity.
Beta-catenin, in addition to its role as a cell adhesion component, also functions as a transcriptional co-activator in the Wnt signaling pathway through its interactions with the family of Tcf and Lef transcription factors (for a review see Polakis, (1999) Current Opinion in Genetics & Development, 9:15-21 and Gat U., et al., (1998) Cell 95:605-614).
Deregulation of beta-catenin signaling is a frequent and early event in the development of a variety of human tumors, including colon cancer, melanoma, ovarian cancer, and prostate cancer. Activation of beta-catenin signaling can occur in tumor cells by loss-of-function mutations in the tumor suppressor genes Axin or APC, as well as by gain-of-function mutations in the oncogene beta-catenin itself. Axin normally functions as a scaffolding protein that binds beta-catenin, APC, and the serine/threonine kinase GSK3-beta. Assembly of this degradation complex allows GSK3-beta to phosphorylate beta-catenin, which leads to beta-catenin ubiquitination and degradation by the proteasome. In the absence of Axin activity, beta-catenin protein becomes stabilized and accumulates in the nucleus where it acts as a transcriptional co-activator with TCF for the induction of target genes, including the cell cycle regulators cyclin D1 and c-Myc.
The APC gene, which is mutant in adenomatous polyposis of the colon, is a negative regulator of beta-catenin signaling (Korinek, V. et al., (1997) Science 275: 1784-1787; Morin, P. J., et al., (1997) Science 275: 1787-1790). The APC protein normally binds to beta-catenin and, in combination with other proteins (including glycogen synthase kinase-3b and axin, is required for the efficient degradation of b-catenin. The regulation of beta-catenin is critical to the tumor suppressive effect of APC and that this regulation can be circumvented by mutations in either APC or beta-catenin.
While mammals contain only a single beta-catenin gene, C. elegans contains three (Korswagen H C, et al., (2000) Nature 406:527-32). Each worm beta-catenin appears to, carry out unique functions (Korswagen H C, et al., (2000) Nature 406:527-32, Nartarajan L et al. (2001) Genetics 159: 159-72). Because of the divergence of function in C. elegans, it is possible to specifically study beta-catenin role in cell adhesion, which is mediated by the C. elegans beta-catenin HMP-2.
The C. elegans gene pry-1 is the structural and functional ortholog of vertebrate Axin (Korswagen H C et al. (2002) Genes Dev. 16:1291-302). PRY-1 is predicted to contain conserved RGS and DIX domains that, in Axin, bind APC and Dishevelled, respectively. Overexpression of the C. elegans pry-1 gene in zebrafish can fully rescue the mutant phenotype of masterblind, the zebrafish Axin 1 mutation. pry-1 loss-of-function mutations produce several phenotypes that appear to result from increased beta-catenin signaling (Gleason J E et al. (2002) Genes Dev. 16:1281-90; Korswagen et al., supra).
Mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4) is a serine-threonine kinase that activates the c-Jun N-terminal kinase signaling pathway, and may be involved in TNF alpha signaling (Yao, Z., et al (1999) J Biol Chem 274:2118-25; Huang, T. T., et al (2000) Proc Natl Acad Sci USA 97:1014-9).
KIAA0551 is a protein with high similarity to mitogen-activated protein kinase kinase kinase kinase 6 (MAP4K6) which activates the JUN N terminal kinase (JNK) and p38 MAP kinase pathways.
Misshapen/NIKs-related kinase (MNK) is a member of the germinal center kinase (GCK) family of kinases. MINK activates the cJun N-terminal kinase and the p38 pathways (Dan, I., et al (2000) FEBS Lett 469:19-23).
The ability to manipulate the genomes of model organisms such as C. elegans provides a powerful means to analyze biochemical processes that, due to significant evolutionary conservation, have direct relevance to more complex vertebrate organisms. Due to a high level of gene and pathway conservation, the strong similarity of cellular processes, and the functional conservation of genes between these model organisms and mammals, identification of the involvement of novel genes in particular pathways and their functions in such model organisms can directly contribute to the understanding of the correlative pathways and methods of modulating them in mammals (see, for example Dulubova I, et al, J Neurochem 2001 April; 77(1):229-38; Cai T, et al., Diabetologia 2001 January; 44(1):81-8; Pasquinelli A E, et al., Nature. 2000 Nov. 2; 408(6808):37-8; Ivanov I P, et al., EMBO J 2000 Apr. 17; 19(8):1907-17; Vajo Z et al., Mamm Genome 1999 October; 10(10):10004). For example, a genetic screen can be carried out in an invertebrate model organism having underexpression (e.g. knockout) or overexpression of a gene (referred to as a “genetic entry point”) that yields a visible phenotype. Additional genes are mutated in a random or targeted manner. When a gene mutation changes the original phenotype caused by the mutation in the genetic entry point, the gene is identified as a “modifier” involved in the same or overlapping pathway as the genetic entry point. When the genetic entry point is an ortholog of a human gene implicated in a disease pathway, such as Rac, axin, and beta-catenin, modifier genes can be identified that may be attractive candidate targets for novel therapeutics.
All references cited herein, including patents, patent applications, publications, and sequence information in referenced Genbank identifier numbers, are incorporated herein in their entireties.